Old and new heparins.

Heparin is an effective, relatively safe, inexpensive parenteral antithrombotic agent widely used in the prevention and treatment of thromboembolic disorders, but it has several limitations such as the marked intra- and inter-patient variability in its anticoagulant response, its poor bioavailability at low doses and its relatively narrow risk to benefit ratio. Low molecular weight heparins ( LMWHs), ultra LMWHs and synthetic pentasaccharides have been developed from heparin to overcome its limitations. The characteristics of these compounds are reviewed along with the description of their approved clinical uses.